Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats


Appendix II.                             885

riecessarily high, piston speed) was demonstrated by Mr. Willans
in 1888, when he got very economical results with a simple
engine greatly by that means. The time of each stroke is then
too short to allow of the temperature changes which cause con-
densation loss, and the expansion is approximately adiabatic
Lagging the cylinder increases its non-conducting capacity, and
is an additional help.

Steam-jacketing is different to lagging in that heat is actually
given to the steam, to assist re-evaporation. In most cases there
is some advantage in its use, though not always a large one. Its
good effect is greatest with simple engines, slow speeds, large
rate of expansion, and wherever considerable condensation would
otherwise occur : it is of less value in compounds and in non-
condensing engines, while at high rotational speeds it is useless.

Superheating the steam, that is raising its temperature above
saturation point before entering the cylinder, is the most direct
and effective means of reducing and even eliminating con-
el ensation loss. This is not achieved by increased boiler heating
surface, nor by improved thermal efficiency due to higher
temperature, both of which false explanations have been made;
but simply by supplying the saturated steam with such heat as
will partly or entirely prevent condensation, thus keeping the
steam dry, or nearly so, throughout the stroke. The curious
anomaly occurs that a 5% extra heat supply will often eliminate
a 20% loss. Him demonstrated a saving of 22% in 1855, arjd
in 1859 Penn and others saved as much as 30% ; but as steam
pressures increased, a reasonable ' superheat' could not be
obtained without danger at the superheater, and the cylinder
lubricants were also burned up, so the process was abandoned
in 1870. A revival has taken place since 1890, the original
objections having been removed by the substitution of petroleum
For animal oils, and safer superheaters having small parts. The
gain varies from 10% to 50% with 50* to 100 superheat re-
spectively, and according to the perfection of the engine itself;
font averages 25%. The effect on the indicator .diagram is to
raise the expansion curve, while lowering the saturation curve due
to fee,d, and thus the dryness fraction approaches a maximum.

Fig. $43 shews the Schmidt superheater, consisting of a coi